1. Field of the Invention.
This invention relates to remote monitoring systems employing telephone links as a transmission medium, and more particularly to a system for providing remote monitoring of the liquid level in liquid storage tanks, such as underground gasoline storage tanks.
2. Description of the Prior Art.
For many years, gasoline stations were manned by full time attendants employed specifically to sell gasoline and other automotive products. One of the attendant's functions was the daily routine of measuring the liquid level in the storage tanks. This procedure was accomplished by opening the fill port of a tank, and lowering a calibrated pole into the tank until it touched the bottom of the tank. The pole was then withdrawn and the liquid level noted on the gauge. The information was then transferred to permanent records. The station manager would order fuel as required to maintain a desired inventory.
With the coming of the age of the fuel shortages, service stations begain to reclassify both to reduce operating costs, and to broaden product lines to increase profitability. More and more, the status of full service stations changed to that of self-service gasoline dispensing operations, with an attendant reduction in personnel, sometimes limited to only a cashier. This reduction in personnel dictated the need for an alternative method for monitoring the supply of gasoline on hand. Concurrently, chain stores, which dispensed food and or other items as a major product line, added self-service other items as a major product line, added self-service gasoline dispensing operations for the convenience of their customers. However, most laws preclude personnel who dispense food from handling fuel in any way, and this has created a new emphasis on gauging systems which do not require physical contact with the liquid.
The most common type of gauging system is referred to as a bubbler system. In this system compressed air is introduced in very small quantities into a tube which runs from the metering area to the gasoline tank where the tube is extended vertically through the tank to within an inch of the bottom of the tank. The air bubbles out of the bottom of the tube, displacing the gasoline, and the resultant pressure in the tube is proportional to the depth of the fluid in the tank. This pressure is sensed by a pressure gauge which provides a readout indicating the depth of gasoline in inches. One such system is marketed by Murphy Tank Company, of Tulsa, Okla. Another system of this type is marketed by Hersey Products, Inc., of Spartanburg, S.C. These systems are accepted as industry standards.
Although gauging systems are used for monitoring of available fuel in storage tanks in gasoline manufacturing plants, the gauging units associated with storage tanks are generally permanently connected to inventory control with in the plant. Such arrangement is not practical for remote monitoring of fuel inventories of self-service gasoline stations, or the like, due to the large number of self-service stations and the diverse locations of such stations. The installation of direct readout lines between gauging units at each of the locations and a readout unit at a central inventory control center would be prohibitive from a cost standpoint.
A further consideration is that frequently, storage capacity is not utilized effectively, particularly when the station manager is responsible for inventory control. This is in part because the station manager simply does not order fuel when he has the storage capacity. Also, in some instances the supplier cannot deliver needed fuel in time because the manager waits until the last possible minute before ordering the fuel. The additional cost resulting from making special overtime deliveries, as well as lost sales in the event of a run out of fuel, are of major concern to all of the oil companies.